What does MSID in SDP mean?

In WebRTC, the MSID corresponds to the ID of a MediaStream or WebRTC MediaStream (WMS). As mentioned within our MST article a MediaStream can be composed of zero or many MediaStreamTracks, where one MediaStreamTrack corresponds to each of the media sources or channels.To better understand how MediaStreamTracks are synchronized within callstats.js you can read this blogpost.

In a WebRTC-compatible SDP description, all SSRCs intending to be sent from one peer will be identified in the SDP generated by the entity.You can read more about SSRC in this article.

The purpose of the MSID attribute in SDP is to group channels or SSRCs that belongs to the same MediaStream, so if a peer sends two streams, one for audio and one for video, the remote peer will know that 2 separate streams were sent and not a single stream with audio and video.

The number of streams and tracks in the sender is preserved in the receiver.

You can look at the following extract of an SDP message generated at one peer: